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c++loopswhile-loopinitializationdeclaration
Shadi
Shadi

Reputation: 1771

c++ declaration and initialization variables inside while loops

In the following code:

#include <iostream>

using namespace std;

int main()
{
    int num = 0;

    while (num >= 0 && num <= 3)
    {
        int inner_loop_count = 0;
        cout << "Loop # " << ++inner_loop_count << "\n";
        num++;
    }
}

The output is:

Loop # 1
Loop # 1
Loop # 1
Loop # 1

My understanding that the loop scope is between the braces {} and cannot be used to define a loop counter, because the declaration and the initialization will be redone each time.

I tried the following solution:

  1. using static keyword. (Regardless of goodness or badness).
  2. including while itself inside outer {} and declare the counter variable there just right before while.

Question #1: Is my understanding correct?
Question #2: Are there any other -technically- possible solutions?

Thanks

Upvotes: 3

Views: 30210

Answers (5)

Sean
Sean

Reputation: 62472

Your understanding is correct. Another possible solution is to use a for loop:

using namespace std;

int main()
{
    for(int num = 0, inner_loop_count = 1; num <=3; num++, inner_loop_count++)
    {
        cout << "Loop # " << inner_loop_count << "\n"; 
    }

    return 0;
}

Upvotes: 1

eerorika
eerorika

Reputation: 238311

A1. Yes, your understanding is spot on. An automatic variable is local to its block (delimited by {}), so it cannot be used as a loop counter if declared within the loop body.

A2. Your 2. suggestion is the typical way to use a counter in a while loop, although isolating the counter with an outer block is usually not necessary. Another possibility: for loop control structure, which does exactly the same, with different syntax (i.e. it is syntactic sugar).

Also, you can use a global variable - which is quite similar to using a local static variable.

Upvotes: 0

JoakimE
JoakimE

Reputation: 1864

My understanding that the loop scope is between the braces {} and cannot be used to define a loop counter, because the declaration and the initialization will be redone each time.

You are correct

You can declare you counter outside your while loop like this:

int inner_loop_count = 0;
while (num >= 0 && num <= 3)
{
    cout << "Loop # " << ++inner_loop_count << "\n"; //beware..Counts +1 before loop is done. First looping will count as 1, but is actually number 0
    num++;
}

Upvotes: 0

Cheers and hth. - Alf
Cheers and hth. - Alf

Reputation: 145249

You can't practically define a loop counter inside the loop body. A static could work technically in a given context, as you mention. But if the loop was entered a second time that counter would not start at 0.

So instead, use a for loop.

That's what it's “for”:

for( int num = 0; num <= 3; ++num )
{
    // ...
}

It's defined by equivalence with a while loop placed in an enclosing braces block where the int num = 0 declaration is placed.

The update ++num is placed at the bottom of the loop body in that equivalent, like this:

// Equivalent:
{
    int num = 0;
    while( num <= 3 )
    {
        // ...
        ++num;
    }
}

… which you avoid having to write by using the for.

Upvotes: 2

Ilya Kobelevskiy
Ilya Kobelevskiy

Reputation: 5345

Your understanding is correct, and another possible solution would be to declare variable outside of a loop:

int inner_loop_count = 0;
while (num >= 0 && num <= 3)
{

Upvotes: 1

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